Integrated apparatus for aromatics production

ABSTRACT

Enabling a transalkylation process to handle both C 10  alkylaromatics and unextracted toluene permits the following improvements to be realized. No longer extracting toluene allows a reformate-splitter column to be eliminated. The extraction unit can be moved to the overhead of a benzene column and integrated together with the transalkylation unit to reduce costs. No longer requiring a rigorous split between C 9  and C 10  alkylaromatics allows a heavy aromatics column to be eliminated. Such an enabled transalkylation process requires stabilization of a transalkylation catalyst through the introduction of a metal function. These improvements result in an aromatics complex apparatus with savings on inside battery limits curve costs and an improvement on the return on investment in such a complex.

FIELD OF THE INVENTION

This invention relates to an aromatics complex flow scheme, which is acombination of apparatus zones using process units that can be used toconvert naphtha into basic petrochemical intermediates of benzene,toluene, and xylene. Based on a metal catalyzed transalkylation processthat handles unextracted toluene and heavier aromatics and an olefinsaturation process, the improved flow scheme removes items of equipmentand processing steps, such as a reformate splitter column and a heavyaromatics column, resulting in significant economic benefits whenproducing xylene isomers.

BACKGROUND OF THE INVENTION

Most new aromatics complexes are designed to maximize the yield ofbenzene and para-xylene. Benzene is a versatile petrochemical buildingblock used in many different products based on its derivation includingethylbenzene, cumene, and cyclohexane. Para-xylene is also an importantbuilding block, which is used almost exclusively for the production ofpolyester fibers, resins, and films formed via terephthalic acid ordimethyl terephthalate intermediates. Accordingly, an aromatics complexmay be configured in many different ways depending on the desiredproducts, available feedstocks, and investment capital available. A widerange of options permits flexibility in varying the product slatebalance of benzene and para-xylene to meet downstream processingrequirements.

A prior art aromatics complex flow scheme has been disclosed by Meyersin the HANDBOOK OF PETROLEUM REFINING PROCESSES, 2d. Edition in 1997 byMcGraw-Hill.

U.S. Pat. No. 3,590,092 to Uitti et al discloses a method for extractingbenzene using a combination of extractive distillation, aromaticside-cut rectification, and fractionation.

U.S. Pat. No. 3,996,305 to Berger discloses a fractionation schemeprimarily directed to transalkylation of toluene and C₉alkylaromatics inorder to produce benzene and xylene. The transalkylation process is alsocombined with an aromatics extraction process. The fractionation schemeincludes a single column with two streams entering and with threestreams exiting the column for integrated economic benefits.

U.S. Pat. No. 4,053,388 to Bailey discloses a process for preparingaromatics from naphtha that achieves an increased yield by integrating acatalytic reforming unit with a thermal hydrocracking unit. Thearomatics are recovered in a complex flow scheme using extractivedistillation, transalkylation, para-xylene separation, and xyleneisomerization processes. A rerun column for heavy aromatics is alsodisclosed.

U.S. Pat. No. 4,341,914 to Berger discloses a transalkylation processwith recycle of C₁₀ alkylaromatics in order to increase yield of xylenesfrom the process. The transalkylation process is also preferablyintegrated with a para-xylene separation zone a xylene isomerizationzone operated as a continuous loop receiving mixed xylenes from thetransalkylation zone feedstock and effluent fractionation zones.

U.S. Pat. No. 4,642,406 to Schmidt discloses a high severity process forxylene production that employs a transalkylation zone thatsimultaneously performs as an isomerization zone over a nonmetalcatalyst. High quality benzene is produced along with a mixture ofxylenes, which allows para-xylene to be separated by absorptiveseparation from the mixture with the isomer-depleted stream being passedback to the transalkylation zone.

U.S. Pat. No. 5,417,844 to Boitiaux et al discloses a process for theselective dehydrogenation of olefins in steam cracking petrol in thepresence of a nickel catalyst and is characterized in that prior to theuse of the catalyst, a sulfur-containing organic compound isincorporated into the catalyst outside of the reactor prior to use.

U.S. Pat. No. 5,658,453 to Russ et al discloses an integrated reformingand olefin saturation process. The olefin saturation reaction uses amixed vapor phase with addition of hydrogen gas to a reformate liquid incontact with a refractory inorganic oxide containing preferably aplatinum-group metal and optionally a metal modifier.

U.S. Pat. No. 5,763,720 to Buchanan et al discloses a transalkylationprocess for producing benzene and xylenes by contacting a C₉ ⁺alkylaromatics with benzene and/or toluene over a catalyst comprising azeolite such as ZSM-12 and a hydrogenation noble metal such as platinum.Sulfur or steam is used to treat the catalyst.

U.S. Pat. No. 5,847,256 to Ichioka et al discloses a process forproducing xylene from a feedstock containing C₉alkylaromatics with theaid of a catalyst with a zeolite that is preferably mordenite and with ametal that is preferably rhenium.

SUMMARY OF THE INVENTION

An aromatics complex flow scheme with an enabled transalkylation processrequires stabilization of a transalkylation catalyst through theintroduction of a metal function. Enabling a transalkylation process tohandle both C₁₀ alkylaromatics and unextracted toluene permits thefollowing flow scheme improvements to be realized. By using toluenewithout first passing it to an extraction unit, the flow scheme omits areformate-splitter column. The concomitantly smaller capacity extractionunit is moved to the overhead of a benzene column. By only extractingbenzene, simple extractive distillation is used, since a more expensivecombined liquid—liquid extraction method is only required for heaviercontaminants. By using both C₉ and C₁₀ alkylaromatics in an enabledtransalkylation unit, the flow scheme further omits a heavy aromaticscolumn.

Another embodiment of the present invention comprises an apparatus thatis based on the process steps, which efficiently converts naphtha intopara-xylene. A transalkylation column may be used to eliminate theseparate benzene column. A xylene column may be used with a side-cut orside-draw conduit instead of a separate heavy aromatics column.

Additional objects, embodiments and details of this invention can beobtained from the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an aromatics complex flow scheme of the present invention,which includes olefin saturation and a metal stabilized transalkylationcatalyst.

FIG. 2 shows an alternate embodiment of the present invention, whichincludes a flow scheme based around a transalkylation stripper columnwith a stabilizer section.

DETAILED DESCRIPTION OF THE INVENTION

Feed to the complex may be naphtha, but can also be pygas, importedmixed xylene, or imported toluene. Naphtha fed to an aromatics complexis first hydrotreated to remove sulfur and nitrogen compounds to lessthan about 0.5 wt-ppm before passing the treated naphtha on to areforming unit 13. Naphtha hydrotreating occurs by contacting naphtha ina line 10 with a naphtha hydrotreating catalyst under naphthahydrotreating conditions in a unit 11. Note that the term “unit” will beused throughout the description herein to refer to various process zonesand such a “zone” may be understood as including process equipment andapparatus pieces such as reactor vessels, heaters, separators,exchangers, piping, pumps, compressors, controllers and any and allother equipment and machinery necessary to perform each process withoutlimitation and so understood to be part of such units or zones by one ofordinary skill in the art of each process.

The naphtha hydrotreating catalyst is typically composed of a firstcomponent of cobalt oxide or nickel oxide, along with a second componentof molybdenum oxide or tungsten oxide, and a third component inorganicoxide support, which is typically a high purity alumina. Generally goodresults are achieved when the cobalt oxide or nickel oxide component isin the range of about 1 to about 5 wt-% and the molybdenum oxidecomponent is in the range of about 6 to about 25 wt-%. The alumina (oraluminum oxide) is set to balance the composition of the naphthahydrotreating catalyst to sum all components up to 100 wt-%. Onehydrotreating catalyst for use in the present invention is disclosed inU.S. Pat. No. 5,723,710, the teachings of which are incorporated hereinby reference. Typical hydrotreating conditions include a liquid hourlyspace velocity (LHSV) from about 1.0 to about 5.0 hr⁻¹, a ratio ofhydrogen to hydrocarbon (or naphtha feedstock) from about 50 to about135 Nm³/m³, and a pressure from about 10 to about 35 kg/cm².

In the reforming unit 13, paraffins and naphthenes are converted toaromatics. This is the only unit in the complex that actually createsaromatic rings. The other units in the complex separate the variousaromatic components into individual products and convert variousaromatic species into higher-value products. The reforming unit 13 isusually designed to run at very high severity, equivalent to producingabout 100 to about 106 Research Octane Number (RON) gasoline reformate,in order to maximize the production of aromatics. This high severityoperation also extinguishes virtually all nonaromatic impurities in theC₈ ⁺ fraction of reformate, and eliminates the need for extraction ofthe C₈ and C₉ aromatics.

In the reforming unit 13, hydrotreated naphtha from a line 12 iscontacted with a reforming catalyst under reforming conditions. Thereforming catalyst is typically composed of a first componentplatinum-group metal, a second component modifier metal, and a thirdcomponent inorganic-oxide support, which is typically high purityalumina. Generally good results are achieved when the platinum-groupmetal is in the range of about 0.01 to about 2.0 wt-% and the modifiermetal component is in the range of about 0.01 to about 5 wt-%. Thealumina is set to balance the composition of the naphtha hydrotreatingcatalyst to sum all components up to 100 wt-%. The platinum-group metalis selected from platinum, palladium, rhodium, ruthenium, osmium, andiridium. The preferred platinum-group metal component is platinum. Themetal modifiers may include rhenium, tin, germanium, lead, cobalt,nickel, indium, gallium, zinc, uranium, dysprosium, thallium, andmixtures thereof. One reforming catalyst for use in the presentinvention is disclosed in U.S. Pat. No. 5,665,223, the teachings ofwhich are incorporated herein by reference. Typical reforming conditionsinclude a liquid hourly space velocity from about 1.0 to about 5.0 hr⁻¹,a ratio of hydrogen to hydrocarbon from about 1 to about 10 moles ofhydrogen per mole of hydrocarbon feed entering the reforming zone, and apressure from about 2.5 to about 35 kg/cm². Hydrogen produced in thereforming unit 13 exits in a line 14.

The reformate product from the reforming unit 13 in a line 15 is sent toa debutanizer zone 53, which typically comprises a debutanizer column 20that strips off the light end hydrocarbons (butanes and lighter) in aline 21. The debutanizer zone 53 may also comprise at least one olefinsaturation zone 16, which may be placed upstream or downstream from thedebutanizer column 20. FIG. 1 illustrates an upstream option while FIG.2 illustrates a downstream option. Moreover, streams from other units inthe aromatics complex may also be sent via a line 19 to the debutanizercolumn 20 for stripping. These other units include the transalkylationzone, which sends a transalkylation stripper-overhead stream in a line17, and the isomerization zone, which sends a deheptanizer overheadstream in a line 18. Both of these units are described in greater detailbelow.

The olefin saturation zone 16 may consist of the well-known claytreating means or other means to treat residual olefin contaminants.Clay treating means includes the optional use of hydrogen as an olefinsaturation catalyst means. Accordingly, the olefin saturation zone 16comprises an olefin saturation catalyst operating under olefinsaturation conditions.

Suitable olefin saturation catalysts in the present invention containelemental nickel or a platinum-group component preferably supported onan inorganic oxide support, which is typically alumina. In the casewhere the elemental nickel is present on a support, the nickel ispreferably present in an amount from about 2 to about 40 wt-% of thetotal catalyst weight. One catalyst for use in the present invention isdisclosed in U.S. Pat. No. 5,658,453, the teachings of which areincorporated herein by reference. Alternatively, clay itself is apreferred olefin saturation catalyst, optionally used with hydrogen, andsuch clay may be defined as a common earth of various colors, compactand brittle when dry, but plastic and tenacious when wet. Clay is ahydrous aluminum silicate generally mixed with powdered feldspar,quartz, sand, iron oxide and various other minerals and is formed fromthe decomposition of aluminous rocks such as feldspar in granite. Anysuitable clay which demonstrates the ability to selectively saturateolefins may be utilized in the present invention. Highly preferred claysinclude attapulgus clay and Montmorillonite clay. It is believed thatthe natural iron content of many types of clay contributes to theability of clay to selectively saturate the olefin compounds in anaromatic feedstock while preserving the aromatic compounds. Typicalolefin saturation conditions include a temperature from about 20° toabout 200° C., a pressure from about 5 to about 70 kg/cm² and astoichiometric ratio of hydrogen, if present, to olefins from about0.1:1 to about 15:1.

The debutanized reformate comprising aromatics in a line 22 is combinedwith a transalkylation stripper-bottoms stream in a line 24 and sent toa benzene-toluene (BT) fractionation zone 54 via a line 23. The BTfractionation zone 54 generally comprises at least one column, andusually comprises a benzene column 25 and a toluene column 31. However,the benzene column 25 may be eliminated in favor of a transalkylationstripper column 52, with a stabilizer section sufficient to produce asuitable benzene stream as shown in FIG. 2. The BT fractionation zone 54produces a benzene-enriched stream in a line 26, a toluene-enrichedstream in a line 32, and a xylene-plus-enriched stream in a line 33.Typically, the benzene-enriched stream in the line 26 is produced fromthe overhead of the benzene column 25, with the bottom of the benzenecolumn 25 being sent via a line 30 to feed the toluene column 31. Thetoluene-enriched stream in the line 32 is produced from the overhead ofthe toluene column 31 and sent to a transalkylation unit 36, with thebottom of the toluene column 31 producing the xylene-plus-enrichedstream in the line 33. The xylene-plus-enriched stream in the line 33from the bottom of the toluene column 31 is sent to a xylene recoverysection 55 of the aromatics complex described below.

The benzene-enriched stream in the line 26 is sent to an extractivedistillation zone 27 which produces a high purity benzene product streamin a line 29 and rejects a by-product raffinate stream in a line 28. Theraffinate stream may be blended into gasoline, used as feedstock for anethylene plant, or converted into additional benzene by recycling to thereforming unit 13. The use of extractive distillation instead ofliquid—liquid extraction or combined liquid—liquid extraction/extractivedistillation processes results in an economic improvement. Theextractive distillation zone 27 will generally comprise at least onecolumn known as a main distillation column and may comprise a secondcolumn known as a recovery column. The second column may also be foundby re-using a benzene column from another fractionation part of thearomatics complex such as BT fractionation zone 54.

Extractive distillation is a technique for separating mixtures ofcomponents having nearly equal volatility and having nearly the sameboiling point. It is difficult to separate the components of suchmixtures by conventional fractional distillation. In extractivedistillation, a solvent is introduced into a mainextractive-distillation column above the entry point of thehydrocarbon-containing fluid mixture that is to be separated. Thesolvent affects the volatility of the hydrocarbon-containing fluidcomponent boiling at a higher temperature differently than thehydrocarbon-containing fluid component boiling at a lower temperaturesufficiently to facilitate the separation of the varioushydrocarbon-containing fluid components by distillation and such solventexits with the bottoms fraction. Suitable solvents includetetrahydrothiophene 1,1-dioxide (or sulfolane), NFM(n-formylmorpholine), NMP (n-methylpyrrolidone), diethylene glycol,triethylene glycol, tetraethylene glycol, methoxy triethylene glycol,and mixtures thereof. Other glycol ethers may also be suitable solventsalone or in combination with those listed above. The raffinate stream inthe line 28 comprising nonaromatic compounds exits the extractivedistillation zone 27 overhead of the main extractive-distillationcolumn, while the bottoms fraction containing solvent and benzene exitsbelow the main extractive-distillation column. The bottoms stream fromthe main extractive-distillation column is sent to a solvent-recoverycolumn, where benzene is recovered overhead in the line 29 and thesolvent is recovered from the bottom and passed back to the mainextractive-distillation column. The recovery of high purity benzene inthe line 29 from the extractive distillation zone 27 typically exceeds99 wt-%.

The extractive distillation section of the present invention issimplified in several ways by being free to process a benzene richstream. For example, expensive steam stripping equipment normallynecessary to separate aromatics from the solvent in a solvent-recoverycolumn can be eliminated when processing primarily benzene feeds. Inother words, operating with a substantial absence of steam stripping andrelated equipment is a characteristic of the present invention, andsubstantial absence refers to absent amounts of steam normally neededfor solvent recovery from heavier aromatic mixtures including toluene.Benzene may also used instead of steam to regenerate any solvent neededfor the unit. Note that the main extractive-distillation column, thesolvent-recovery column, and the optional benzene column of extractivedistillation zone 27 are not specifically shown in either of theFigures.

In one simplified flow scheme of the invention, the solvent-recoverycolumn is made redundant to the benzene column 25. Thus thetransalkylation stripper column 52 shown in FIG. 2 still produces thebenzene-enriched stream 26 but now a separate benzene column actsinstead as a recovery column for the extractive distillation unitwhereby the main extractive-distillation column product stream (notshown) containing solvent and benzene can be fractionated to produce ahighly pure benzene product overhead and the solvent can be recoveredfrom the bottoms. Alternatively, and in addition to the flowschemedescribed above, the benzene column can also act in conjunction with asolvent-recovery column to provide effectively two recovery columns andallow increased benzene recovery with additional recovered benzeneproduct and a purified solvent stream.

The toluene-enriched stream in the line 32 is usually blended with astream in a line 41 rich in C₉ and C₁₀ alkylaromatics produced by axylene column 39 and charged via a line 34 to the transalkylation unit36 for production of additional xylenes and benzene. In thetransalkylation unit 36, the feed is contacted with a transalkylationcatalyst under transalkylation conditions. The preferred catalyst is ametal stabilized transalkylation catalyst. Such catalyst comprises asolid-acid component, a metal component, and an inorganic oxidecomponent. The solid-acid component typically is either a pentasilzeolite, which include the structures of MFI, MEL, MTW, MTT and FER(IUPAC Commission on Zeolite Nomenclature), a beta zeolite, or amordenite. Preferably it is mordenite zeolite. Other suitable solid-acidcomponents include mazzite, NES type zeolite, EU-1, MAPO-36, MAPSO-31,SAPO-5, SAPO-11, SAPO-41. Preferred mazzite zeolites include ZeoliteOmega. The synthesis of the Zeolite Omega is described in U.S. Pat. No.4,241,036. European Patent Application EP 0 378 916 A1 describes NEStype zeolite and a method for preparing NU-87. The EUO structural-typeEU-1 zeolite is described in U.S. Pat. No. 4,537,754. MAPO-36 isdescribed in U.S. Pat. No. 4,567,029. MAPSO-31 is described in U.S. Pat.No. 5,296,208 and typical SAPO compositions are described in U.S. Pat.No. 4,440,871 including SAPO-5, SAPO-11, SAPO-41

The metal component typically is a noble metal or base metal. The noblemetal is a platinum-group metal is selected from platinum, palladium,rhodium, ruthenium, osmium, and iridium. The base metal is selected fromthe group consisting of rhenium, tin, germanium, lead, cobalt, nickel,indium, gallium, zinc, uranium, dysprosium, thallium, and mixturesthereof. The base metal may be combined with another base metal, or witha noble metal. Preferably the metal component comprises rhenium.Suitable metal amounts in the transalkylation catalyst range from about0.01 to about 10 wt-%, with the range from about 0.1 to about 3 wt-%being preferred, and the range from about 0.1 to about 1 wt-% beinghighly preferred. Suitable zeolite amounts in the catalyst range fromabout 1 to about 99 wt-%, preferably between about 10 to about 90 wt-%,and more preferably between about 25 to about 75 wt-%. The balance ofthe catalyst is composed of a refractory binder or matrix that isoptionally utilized to facilitate fabrication of the catalyst, providestrength and reduce fabrication costs. The binder should be uniform incomposition and relatively refractory to the conditions used in theprocess. Suitable binders include inorganic oxides such as one or moreof alumina, magnesia, zirconia, chromia, titania, boria, thoria,phosphate, zinc oxide and silica. Alumina is a preferred binder. Onetransalkylation catalyst for use in the present invention is disclosedin U.S. Pat. No. 5,847,256, the teachings of which are incorporatedherein by reference

Conditions employed in the transalkylation zone normally include atemperature of from about 200° to about 540° C. The transalkylation zoneis operated at moderately elevated pressures broadly ranging from about1 to about 60 kg/cm². The transalkylation reaction can be effected overa wide range of space velocities, with higher space velocities effectinga higher ratio of para-xylene at the expense of conversion. Liquidhourly space velocity generally is in the range of from about 0.1 toabout 20 hr⁻¹. The feedstock is preferably transalkylated in the vaporphase and in the presence of hydrogen supplied via a line 35. Iftransalkylated in the liquid phase, then the presence of hydrogen isoptional. If present, free hydrogen is associated with the feedstock andrecycled hydrocarbons in an amount of about 0.1 moles per mole ofalkylaromatics up to about 10 moles per mole of alkylaromatic. Thisratio of hydrogen to alkylaromatic is also referred to as hydrogen tohydrocarbon ratio.

The effluent from the transalkylation unit 36 is sent to thetransalkylation stripper column 52 to remove light ends, then sent tothe BT fractionation zone 54 through the lines 24 and 23. There thebenzene product is recovered, and the xylenes are fractionated out andsent to the xylene recovery section 55 via the xylene plus enrichedstream in the line 33. The overhead material from the transalkylationstripper column 52 is normally recycled back via the line 17 to thereforming unit debutanizer for recovery of residual benzene.Alternatively, a stabilizer section or column is placed on or after thetransalkylation stripper column 52. This transalkylation stabilizersection can produce a benzene-enriched stream suitable for extractivedistillation, and eliminate the need for a separate benzene column inthe BT fractionation section as shown in FIG. 2, which shows section 25on top of column 52. Such a stabilizer or stripper column from thetransalkylation zone is thus alternatively encompassed in the definitionof the BT fractionation zone 54 when the separate benzene column 25 iseliminated. The transalkylation stripper column 52 can also accepttreated product from the olefin saturation zone or overhead from thealkylaromatic isomerization deheptanizer column that would normally berecycled back to the reforming unit debutanizer column 20.

As noted above, the xylene-plus-enriched stream in the line 33 from thebottom of the toluene column 31 is sent to the xylene recovery section55 of the aromatics complex. This section of the aromatics complexcomprises at least one xylene column 39, and generally will furtherinclude a process unit for separation of at least one xylene isomer,which is typically the para-xylene product from the aromatics complexbut may instead be a meta-xylene isomer. Hereinafter the xylene-isomerseparation zone will be described in terms of a para-xylene isomer.Preferably such a para-xylene separation zone 43 is operated inconjunction with an isomerization unit 51 for isomerization of theremaining alkylaromatic compounds back to an equilibrium or nearequilibrium mixture containing para-xylene, which can be recycled aroundagain for further recovery in a loop-wise fashion. Accordingly, thexylene-plus-enriched stream in the line 33, which may be blended with arecycle stream in a line 38 to form a stream in a line 37, is charged tothe xylene column 39. The xylene column 39 is designed to rerun a feedstream in a line 40 to the para-xylene separation zone 43 down to verylow levels of C₉alkylaromatics (A₉) concentration. A₉ compounds maybuild up in a desorbent circulation loop within the para-xyleneseparation zone 43, so it is more efficient to remove this materialupstream in the xylene column 39. The overhead feed stream in the line40 from the xylene column 39 is charged directly to the para-xyleneseparation zone 43.

Material from the lower part of the xylene column 39 is withdrawn as astream rich in C₉ and C₁₀ alkylaromatics via the line 41, which is thensent to the transalkylation zone 36 for production of additional xylenesand benzene. The stream in the line 41 taken as a sidecut stream on thexylene column (which eliminates a heavy aromatics column) is reallyenabled by the metal stabilized transalkylation catalyst. A separatecolumn doing a rigorous split to keep coke precursors such as methylindan or naphthalene out of the stream is no longer needed because themetal stabilized transalkylation catalyst can handle them withoutsignificant deactivation due to coking. Any remaining C₁₁ ⁺ material isrejected from the bottom of the xylene column 39 via a line 42. Anotherembodiment is to just send the whole xylene column bottoms stream to thetransalkylation unit instead of the sidecut stream.

Alternatively, if ortho-xylene is to be produced in the complex, thexylene column is designed to make a split between meta and ortho-xyleneand drop a targeted amount of orthoxylene to the bottoms. The xylenecolumn bottoms is then sent to an ortho-xylene column (not shown) wherehigh purity ortho-xylene product is recovered overhead. Material fromthe lower part of the ortho-xylene column is withdrawn as a stream richin C₉ and C₁₀ alkylaromatics then sent to the transalkylation unit. Anyremaining C₁₁ ⁺ material is rejected from the bottom of the ortho-xylenecolumn.

The para-xylene separation zone 43 may be based on a fractionalcrystallization process or an adsorptive separation process, both ofwhich are well known in the art, and preferably is based on theadsorptive separation process. Such adsorptive separation can recoverover 99 wt-% pure para-xylene in a line 44 at high recovery per pass.Any residual toluene in the feed to the separation unit is extractedalong with the para-xylene, fractionated out in a finishing columnwithin the unit, and then optionally recycled to the transalkylationstripper column 52. Thus, the raffinate from the para-xylene separationzone 43 is almost entirely depleted of para-xylene, to a level usuallyof less than 1 wt-%. The raffinate is sent via a line 45 to thealkylaromatics isomerization unit 51, where additional para-xylene isproduced by reestablishing an equilibrium or near-equilibriumdistribution of xylene isomers. Any ethylbenzene in the para-xyleneseparation unit raffinate is either converted to additional xylenes orconverted to benzene by dealkylation, depending upon the type ofisomerization catalyst used.

In the alkylaromatic isomerization unit 51, the raffinate stream in theline 45 is contacted with an isomerization catalyst under isomerizationconditions. The isomerization catalyst is typically composed of amolecular sieve component, a metal component, and an inorganic oxidecomponent. Selection of the molecular sieve component allows controlover the catalyst performance between ethylbenzene isomerization andethylbenzene dealkylation depending on overall demand for benzene.Consequently, the molecular sieve may be either a zeoliticaluminosilicate or a non-zeolitic molecular sieve. The zeoliticaluminosilicate (or zeolite) component typically is either a pentasilzeolite, which include the structures of MFI, MEL, MTW, MTT and FER(IUPAC Commission on Zeolite Nomenclature), a beta zeolite, or amordenite. The non-zeolitic molecular sieve is typically one or more ofthe AEL framework types, especially SAPO-11, or one or more of the ATOframework types, especially MAPSO-31, according to the “Atlas of ZeoliteStructure Types” (Butterworth-Heineman, Boston, Mass., 3rd ed. 1992).The metal component typically is a noble metal component, and mayinclude an optional base metal modifier component in addition to thenoble metal or in place of the noble metal. The noble metal is aplatinum-group metal is selected from platinum, palladium, rhodium,ruthenium, osmium, and iridium. The base metal is selected from thegroup consisting of rhenium, tin, germanium, lead, cobalt, nickel,indium, gallium, zinc, uranium, dysprosium, thallium, and mixturesthereof. The base metal may be combined with another base metal, or witha noble metal. Suitable total metal amounts in the isomerizationcatalyst range from about 0.01 to about 10 wt-%, with the range fromabout 0.1 to about 3 wt-% preferred. Suitable zeolite amounts in thecatalyst range from about 1 to about 99 wt-%, preferably between about10 to about 90 wt-%, and more preferably between about 25 to about 75wt-%. The balance of the catalyst is composed of inorganic oxide binder,typically alumina. One isomerization catalyst for use in the presentinvention is disclosed in U.S. Pat. No. 4,899,012, the teachings ofwhich are incorporated herein by reference.

Typical isomerization conditions include a temperature in the range fromabout 0° to about 600° C. and a pressure from atmospheric to about 50kg/cm³. The liquid hourly hydrocarbon space velocity of the feedstockrelative to the volume of catalyst is from about 0.1 to about 30 hr⁻¹.The hydrocarbon contacts the catalyst in admixture with a gaseoushydrogen-containing stream in a line 46 at a hydrogen-to-hydrocarbonmole ratio of from about 0.5:1 to 15:1 or more, and preferably a ratioof from about 0.5 to 10. If liquid phase conditions are used forisomerization, then no hydrogen is added to the unit.

The effluent from the isomerization unit 51 is sent via a line 47 to adeheptanizer column 48. A bottoms stream in a line 49 from thedeheptanizer column 48 is treated to remove olefins, if necessary, in anolefin saturation unit 50 with the olefin saturation methods describedabove. An alternative is to put the olefin saturation unit 50 after theisomerization unit 51 and use the deheptanizer column 48 to removeresidual hydrogen. If the catalyst used in the isomerization unit 51 isthe ethylbenzene dealkylation type, then olefin saturation may not berequired at all.

The deheptanizer bottoms stream in the line 49, after olefin treatment,is then recycled back to the xylene column 39 via the line 38. In thisway, all the C₈ aromatics are continually recycled within the xylenesrecovery section of the complex until they exit the aromatics complex aspara-xylene, benzene, or optionally ortho-xylene. The overhead from thedeheptanizer column 48 is normally recycled back via the line 18 to thereforming unit debutanizer column 20 for recovery of residual benzene.Alternatively, the overhead liquid is recycled back to thetransalkylation stripper column 52.

Accordingly, the aromatics complex of the present invention displaysexcellent economic benefits. These improvements result in an aromaticscomplex with savings on inside battery limits curve costs and animprovement on the return on investment in such a complex.

1. An integrated apparatus for producing benzene and xylene isomers froman aromatics rich stream: (a) an extractive-distillation zone comprisinga main-distillation column and a benzene column, wherein abenzene-enriched stream produces a raffinate stream and a benzeneproduct stream which is recovered as a product of said apparatus; (b) afractionation zone comprising a toluene column, wherein the aromaticsrich stream and at least a portion of a transalkylation product streamare separated to produce a toluene-enriched stream, and axylene-plus-enriched stream; (c) a transalkylation zone comprising areactor and a transalkylation stripper column wherein thetoluene-enriched stream and a xylene-column stream rich in C₉ and C₁₀alkylaromatics are contacted with a metal-stabilized transalkylationcatalyst under transalkylation conditions to produce the transalkylationproduct stream of step (b) and the benzene-enriched stream of step (a);and (d) a xylene recovery section comprising a xylene fractionationcolumn and a xylene-isomer separation zone, wherein the xylenefractionation column separates the xylene-plus-enriched stream of step(b) to provide the xylene-column stream rich in C₉ and C₁₀alkylaromatics of step (c) and an overhead xylene stream, and whereinthe xylene-isomer separation zone concentrates the overhead xylenestream into a xylene isomer enriched product stream which is recoveredas a product stream of said apparatus.
 2. The apparatus of claim 1wherein the xylene recovery section further comprises an alkylaromaticsisomerization zone comprising a deheptanizer fractionation zone havingat least one deheptanizer column and optionally at least one olefinsaturation zone.
 3. The apparatus of claim 1 wherein the benzene columnis further characterized as operating without substantial steamstripping equipment.
 4. The apparatus of claim 1 wherein the xylenecolumn is further characterized by having a means for withdrawing as asidecut the stream rich in C₉ and C₁₀ alkylaromatics.
 5. The apparatusof claim 1 wherein the xylene recovery section further comprises anortho-xylene column.
 6. The apparatus of claim 1 wherein the aromaticsrich stream is selected from the group consisting of catalyticreformate, pygas, imported mixed xylenes, imported toluene, and mixturesthereof.
 7. The apparatus of claim 1 wherein the main-distillationcolumn contains a solvent selected from the group consisting ofsulfolane, diethylene glycol, triethylene glycol, tetraethylene glycol,n-formylmorpholine, n-methylpyrrolidone, methoxy triethylene glycol, andmixtures thereof.
 8. The apparatus of claim 7 wherein the benzene columnseparates a main distillation stream exiting the main-distillationcolumn to produce a solvent stream that is returned to themain-distillation column.
 9. The apparatus of claim 8 further comprisinga recovery column which accepts at least a part of the solvent streamand produces a recovered benzene stream and a purified solvent streamthat is passed to the main-distillation column.
 10. An integratedapparatus for producing benzene and xylene isomers from an aromaticsrich stream comprising: (a) a fractionation zone comprising atransalkylation stripper column and a toluene column, wherein thearomatics rich stream is provided along with at least a portion of atransalkylation product stream and separated to produce abenzene-enriched stream, a toluene-enriched stream, and axylene-plus-enriched stream; (b) an extractive-distillation zonecomprising a main distillation column and a recovery column, wherein thebenzene-enriched stream is provided along with a solvent selected fromthe group consisting of sulfolane, diethylene glycol, triethyleneglycol, tetraethylene glycol, n-formylmorpholine, n-methylpyrrolidone,methoxy triethylene glycol, and mixtures thereof, to the maindistillation column that produces a raffinate stream and a maindistillation stream that is provided to the recovery column operatingwithout substantial steam stripping that produces at least a portion ofthe solvent and a benzene product which is recovered as a product streamfrom said apparatus; (c) a xylene column, wherein a xylene-column streamrich in C₉ and C₁₀ alkylaromatics is produced as a sidecut from thexylene column; (d) a transalkylation zone comprising a reactor, whereinthe toluene-enriched stream is provided with the xylene-column streamrich in C₉ and C₁₀ alkylaromatics and contacted with a metal-stabilizedtransalkylation catalyst under transalkylation conditions to produce thetransalkylation product stream of step (a); and (e) a xylene recoverysection comprising a xylene fractionation zone and a xylene-isomerseparation zone, wherein the xylene fractionation zone comprises thexylene column of step (c) that separates the xylene-plus-enriched streamof step (a) into the xylene-column stream rich in C₉ and C₁₀alkylaromatics and an overhead xylene stream, and wherein thexylene-isomer separation zone concentrates the overhead xylene streaminto a xylene isomer enriched product stream which is recovered as aproduct stream of said apparatus.
 11. The apparatus of claim 10 whereinthe xylene recovery section further comprises an alkylaromaticsisomerization zone.
 12. The apparatus of claim 11 wherein alkylaromaticsisomerization zone further comprises a deheptanizer fractionation zonehaving at least one deheptanizer column and optionally at least oneolefin saturation zone.
 13. The apparatus of claim 10 wherein the xylenerecovery section further comprises an ortho-xylene column.
 14. Theapparatus of claim 10 wherein the xylene-isomer separation zone is anadsorptive separation process or a fractional crystallization process.15. The apparatus of claim 10 wherein the aromatics rich stream isselected from the group consisting of catalytic reformate, pygas,imported mixed xylenes, imported toluene, and mixtures thereof.